Natural gas production and consumption will continue to expand in the foreseeable future in the United States and throughout the world. However, the processing and delivery of natural gas is complicated by the fact that the material is typically saturated with water vapor when extracted from underground sources. Accordingly, natural gas must be dehydrated before it enters the delivery pipeline.
Among the known natural gas dehydration methods, the glycol absorption process has gained wide acceptance because of its effectiveness, reliability, and relatively low cost. However, recent concern about the environmental impact from the release of benzene and other toxic hydrocarbons to the atmosphere as a result of the use of the glycol absorption process militates against the expansion of the use of the glycol dehydration process.
A less environmentally hazardous process for the dehydration of gaseous materials involves the use of membrane separation techniques. However, membrane separation processes are generally only competitive for processing sub-quality natural gas to remove other undesirable gaseous components along with the water vapor due to the inherent loss of natural gas in the process and its higher cost.
Another dehydration method includes the use of a desiccant or molecular sieve material to absorb the water vapor and thus dry the gaseous material. Once the absorption efficiency of the desiccant or molecular sieve material decreases to a set limit, the material is either replaced or regenerated, typically with heat to remove the water. While such systems are mostly suitable for compressed air and non-flammable gaseous materials, dehydration of natural gas using such a system is more difficult and often hazardous. Natural gas may contain entrained organic liquids which may deposit on the desiccant or molecular sieve material. Accordingly, handling or disposing of the used desiccant containing flammable hydrocarbons is a safety concern and should be addressed in a suitable manner.
Refrigeration processes have also been used to reduce the water vapor content of gaseous materials. In the commercially practiced refrigeration processes, the entire gaseous stream is cooled and the water vapor is deposited as condensate and/or as ice on the cold surfaces of the refrigeration system. A problem with existing refrigeration processes is that the ice tends to build up and plug the flow areas of the system. A portion of the ice and hydrates of the gaseous material may also be entrained in the gas stream and must be separated upon exit from the system. Hence, the principal difficulty associated with refrigeration processes is the accumulation and removal of solid deposits from the tubes and removal of hydrates and entrained solids from the gas exiting the refrigeration system. Furthermore, the thermal efficiency of a refrigeration process is often low because the entire gaseous material must be cooled and because the accumulation of deposits on the cold surfaces impair the heat transfer efficiency of the system. As a result, the equipment for refrigeration processes is bulky and often requires the use of two identical units which are operated alternatively in order to melt ice deposits. Because of the problems and limitations associated with currently available refrigeration systems, these processes have failed to gain wide acceptance for use in dehydrating natural gas.
An attempt to improve the refrigeration process is described in U.S. Pat. No. 2,475,255 to W. F. Rollman. In his process, Rollman uses fluidized chilled granular materials to cool a gas, freeze the moisture, and abrade the ice deposited on the chilled wall surfaces of a refrigeration system. Ice particles which are entrained in the exiting gaseous stream from the refrigeration system are separated by use of a cyclone. The system described by Rollman is quite complicated and cumbersome to start up and operate due to the need to maintain a fluidized stream of solid inert particles and ice at all times during the dehydration sequence. Abrasion and/or erosion of the cooling surfaces may also be a problem with the system described in the '255 patent due to the movement of the solid inert particles and ice through the system.
Accordingly, it is an object of the present invention to provide an apparatus and process for dehydrating a gaseous material.
Another object of the invention is to provide a dehydration system which is environmentally friendly.
A further object of the invention is to provide an apparatus and method for dehydrating a gaseous stream while reducing the amount of entrained solids and/or hydrated gases in the dehydrated stream exiting the apparatus.
Another object of the invention is to provide a simple, efficient, easy to maintain system for dehydrating gases which uses readily available materials.
Still another object of the invention is to provide a system for dehydrating gases which is adaptable for dehydrating a wide variety of gaseous materials.